If the skies are cloudy, go back to bed. Otherwise keep looking.
Before long you'll spot something that makes the trip outside
worthwhile: a bright shooting star -- and a genuine piece of
Halley's comet!

"It's the annual Orionid meteor shower," explains
Bill Cooke, a member of the Space Environments team at the Marshall
Space Flight Center (MSFC). "Every year in October Earth
passes through a stream of dusty debris shed long ago by Halley's
comet." When bits of comet dust -- most no larger than grains
of sand -- strike Earth's atmosphere and disintegrate, they become
"shooting stars."

Above: Artist Duane
Hilton created this picture of an Orionid meteor streaking
over a clump of golden aspens near Bishop, California.

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The Orionids -- so named because they appear to streak
out of a point (called the radiant) in the constellation Orion
-- will peak on Sunday morning, October 21st. Sky watchers north
of the equator with dark clear skies will spot 15 to 20 meteors
each hour before dawn. Observers south of the equator will see
almost as many: 10 to 15 per hour.

Finding the Orionid radiant is easy. It lies near the left
shoulder of Orion the Hunter, roughly centered within an eye-catching
triangle consisting of Sirius -- the brightest star in the sky
-- and the giant planets Jupiter and Saturn. (These stars and
planets are in the southeastern sky before dawn, as viewed from
mid-northern latitudes.)

But don't stare directly at the radiant, say experienced meteor
watchers. Orionids that appear there will seem short and stubby
-- a result of foreshortening. Instead, look toward any dark
region of the sky about 90 degrees away. You'll see just as many
Orionids, but they will seem longer and more dramatic. The tails
of all Orionid meteors, no matter where they appear, will point
back toward the radiant in Orion.

Above: Observers at mid-northern latitudes can find the
radiant of the Orionid meteor shower in the southeastern sky
at 3 a.m. local time on October 21st. Click

The October Orionids are cousins of the eta Aquarids -- a
mostly southern hemisphere meteor shower in May. Both spring
from Halley's comet.

"Earth comes close to the
orbit of Halley's comet twice a year, once in May and again
in October," explains Don Yeomans, manager of NASA's Near-Earth
Object Program at the Jet Propulsion Laboratory. Although the
comet itself is rarely nearby -- it's beyond the orbit of Saturn
now -- Halley's dusty debris constantly moves through the inner
solar system and causes the two regular meteor showers.

In
1986, the last time Comet Halley swung past the Sun, solar heating
evaporated about 6 meters of dust-laden ice from the comet's
nucleus. That's typical, say researchers. The comet has been
visiting the inner solar system every 76 years for millennia,
shedding dust each time.

At first, newly-liberated dust specks simply follow the comet
-- which means they can't strike our planet. Earth's orbit and
Halley's orbit, at their closest points, are separated by 22
million km (0.15 AU). Eventually, though, the dust spreads out
and some of it migrates until it is on a collision course with
Earth.

"Particles that leave the nucleus evolve away from the
orbit of the comet for two main reasons," explains Yeomans.
"First, gravitational perturbations caused by encounters
with planets are different [for the dust and for the comet].
Second, dust particles are affected by solar radiation pressure
to a far greater extent than the comet itself."

"The orbital evolution of Halley's dust is a very complicated
problem," notes Cooke. No one knows exactly how long it
takes for a dust-sized piece of Halley to move to an Earth-crossing
orbit -- perhaps centuries or even thousands of years. However,
one thing is certain: "Orionid meteoroids are old."

And fast. "These meteoroids strike Earth's atmosphere
traveling 66 km/s or 148,000 mph," he continued. Only the
November Leonids (72 km/s) are faster. Such meteors often leave
glowing "trains" (incandescent bits of debris in the
wake of the meteor) that last for several seconds to minutes.

Left:
The European Space Agency's Giotto
probe captured this close-up image of the nucleus of Halley's
comet. The bright jets are spewing debris that might one day
return as Orionid or eta Aquarid meteors. [more]

Cooke and a group of his colleagues, led by Rob Suggs of the
MSFC Engineering Directorate, will be observing the Orionids
this weekend from Huntsville, Alabama, using an array of image-intensified
cameras that can detect stars as faint as 8th magnitude.
(For comparison, the unaided human eye can see stars of 6th magnitude
against a very dark sky. An 8th magnitude star is 6.3 times dimmer
than a 6th magnitude star.)

"This is our tune-up for the Leonid
meteor storm next month," says Suggs. "We plan
to station these cameras, which were developed at the University
of Western Ontario, all around the world to monitor meteor activity
on November 18th." That's when Earth will pass through
a series of debris streams from periodic comet Tempel-Tuttle,
perhaps unleashing a meteor shower of thousands of shooting stars
per hour.

"The Orionids won't produce nearly as many meteors as
the Leonids," added Suggs, "but, like the Leonids,
the Orionids are fast, so they'll provide a good test for our
system."

Next week Science@NASA will feature the results of
Suggs' weekend meteor filming and explain more about the upcoming
Leonid meteor shower. But why wait? You can enjoy a Leonid tune-up
of your own this weekend. Simply go outside, look up, and watch
as Halley's Comet returns ... in bits and pieces.

Editor's Note: Unlike most annual meteor showers the
Orionids do not exhibit a sharp maximum. Meteor rates are elevated
for a few days centered on October 21st. If Sunday morning is
not convenient, try watching the Orionids before dawn on Saturday
or Monday instead.

Web Links

The
Orionids
-- more information including a detailed history of the shower
from Gary Kronk. The Orionids are cousins of the eta
Aquarids.